One of the nice features introduced with Office 2007 and 2010 is that of Live Preview.
With Live Preview, gone are the days of making formatting changes to your document, hitting ‘Apply’ and hoping for the best. Now with Live Preview you can see the effects of your changes before they happen.

First you will need a document with some text in it.

Make sure the ‘Home’ tab is open

Hover your mouse over any one of the styles in the Home tab.

You should now see what your document would look like if you apply that style, if you move the cursor to another style the preview will change accordingly.

This feature also works with selected text and things like Font types and sizes, simply select any text you want to change and then open the font menu, and hover your mouse to see the effect of any change.